The correct TiO2 contunnite structure?

[HexagonReversal]

Hi all,

I have tried to make the cif myself based on the information in the paper, but I’m not very sure if I’m getting it right. Here is my file:

data_TiO2

_chemical_name_common ‘TiO2_Cotunnite’
_cell_length_a 5.163000
_cell_length_b 2.989000
_cell_length_c 5.966000
_cell_angle_alpha 90.000000
_cell_angle_beta 90.000000
_cell_angle_gamma 90.000000
_cell_volume 92.068553
_space_group_name_H-M_alt ‘P n m a’
_space_group_IT_number 62

loop_
_space_group_symop_operation_xyz
‘x, y, z’
‘-x, -y, -z’
‘-x+1/2, -y, z+1/2’
‘x+1/2, y, -z+1/2’
‘-x, y+1/2, -z’
‘x, -y+1/2, z’
‘x+1/2, -y+1/2, -z+1/2’
‘-x+1/2, y+1/2, z+1/2’

loop_
_atom_site_label
_atom_site_occupancy
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
_atom_site_type_symbol
Ti 1.0 0.264000 0.250000 0.110000 Ti
O1 1.0 0.346000 0.250000 0.422000 O
O2 1.0 0.012000 0.750000 0.325000 O

I can pass this file to findsym, which gives me another cif with the atom coordinates quite different. I can see the Ti atom has an equivalent position with the original cif, but I cannot figure out why O1 has its new x-coordinate 0.654 (which does not follow the (x+1/2,1/4,-z+1/2) rule on Bilbao server).

Ti1 Ti 4 c 0.73600 0.25000 0.39000 1.00000 Dx,0,Dz
O1 O 4 c 0.65400 0.25000 0.07800 1.00000 Dx,0,Dz
O2 O 4 c 0.01200 0.25000 0.82500 1.00000 Dx,0,Dz

Another reason why I don’t feel my cif file is correct is that the calculated bulk modulus is very far from the reported value. This is one of an example pw.x input:

&CONTROL
calculation = ‘scf’
outdir = ‘./out/2/’
prefix = ‘TiO2’
pseudo_dir = ‘./’
tprnfor = .true.
tstress = .true.
verbosity = ‘low’
/
&SYSTEM
ibrav = 0
A = 5.163
nat = 12
ntyp = 2
ecutrho = 400
ecutwfc = 50
input_dft = ‘pbe’
occupations = ‘smearing’
smearing = ‘mv’
degauss=0.005d0
/
&ELECTRONS
conv_thr = 1d-08
mixing_beta = 0.7d0
/
CELL_PARAMETERS {alat}
1.000000000000000 0.000000000000000 0.000000000000000
0.000000000000000 0.578926980437730 0.000000000000000
0.000000000000000 0.000000000000000 1.155529730776680
ATOMIC_SPECIES
Ti 47.86700 ti_pbe_v1.4.uspp.F.UPF
O 15.99900 O.pbe-n-kjpaw_psl.0.1.UPF
ATOMIC_POSITIONS {crystal}
Ti 0.264000000000000 0.250000000000000 0.110000000000000
Ti 0.236000000000000 0.750000000000000 0.610000000000000
Ti 0.736000000000000 0.750000000000000 0.890000000000000
Ti 0.764000000000000 0.250000000000000 0.390000000000000
O 0.346000000000000 0.250000000000000 0.422000000000000
O 0.154000000000000 0.750000000000000 0.922000000000000
O 0.654000000000000 0.750000000000000 0.578000000000000
O 0.846000000000000 0.250000000000000 0.078000000000000
O 0.012000000000000 0.750000000000000 0.325000000000000
O 0.488000000000000 0.250000000000000 0.825000000000000
O 0.988000000000000 0.250000000000000 0.675000000000000
O 0.512000000000000 0.750000000000000 0.175000000000000
K_POINTS automatic
6 6 6 0 0 0

and the E-V file (1st column volume in au^3, 2nd column E in Ry):
5.591784000000000106e+02 -8.111739579400000366e+02
5.902439000000000533e+02 -8.113460464400000092e+02
6.213093999999999824e+02 -8.114591772500000388e+02
6.523748000000000502e+02 -8.115259925599999633e+02
6.834402999999999793e+02 -8.115562971900000093e+02

and the ev.x fitting results:
# equation of state: birch 1st order. chisq = 0.2447D-10
# V0 = 700.31 a.u.^3, k0 = 2715 kbar, dk0 = 4.17 d2k0 = 0.000 emin = -811.56021
# V0 = 103.77 Ang^3, k0 = 271.5 GPa

##########################################################################
# Vol. E_calc E_fit E_diff Pressure Enthalpy
# a.u.^3 Ry Ry Ry GPa Ry
##########################################################################
559.18 -811.17396 -811.17396 -0.00000 97.90 -807.45259
590.24 -811.34605 -811.34605 0.00000 66.39 -808.68224
621.31 -811.45918 -811.45917 -0.00001 41.73 -809.69652
652.37 -811.52599 -811.52600 0.00001 22.32 -810.53606
683.44 -811.55630 -811.55630 -0.00000 6.97 -811.23270

Soooo I get 271.5 GPa while the paper is reporting 431 GPa. There are many possible reasons I could get wrong (psuedopotential, k-mesh etc.) and I have no idea why I get this.

The very first thing I want to verify that my cif is correct. Can anyone help?

Thank you so much!

[Yan_DFT]

in my calculation, i also got the bulk modulus 266.7 GPa for cotunnite structure of TiO2, there are several reasons, most suspicious of all, they adopted ab initio LCAO-HF and FPLMTO simulations, which may generate different results compared to ‘pbe’ calculation .

[Author]

Posts